1. Field of the Invention
Embodiments of the invention relates to a display device, and more particularly, to a liquid crystal display device and a method of driving the same. Although embodiments of the invention are suitable for a wide scope of applications, they are particularly suitable for obtaining a liquid crystal display device having a backlight unit that automatically adjusts according to ambient brightness and a method of driving the same.
2. Discussion of the Related Art
Recently, a display device has become thinner with larger display area as an industrial utilization increases. Among the various types of flat panel display (“FPD”) devices, liquid crystal display (“LCD”) devices and plasma display panel (“PDP”) devices are widely used.
A PDP device is an self-emissive type display device where light is emitted from plasma of fluorescent materials in a sidewall between two substrates according to an applied voltage. In contrast, an LCD device is a non-emissive type display device where images are displayed by adjusting light from a backlight unit with a liquid crystal layer as a shutter. Since grey levels are displayed by a digital voltage in a PDP device, the PDP device has a disadvantage in displaying natural images. On the contrary, since an analog voltage is applied to both sides of a liquid crystal layer in an LCD device, the LCD device displays a natural image as compared with a PDP device.
Among LCD devices, an active matrix liquid crystal display (“AMLCD”) device is widely used. In an AMLCD device, a thin film transistor (“TFT”) is connected to a pixel and adjusts a voltage level of the pixel as a switching element to change light transmittance of the pixel and display images.
FIG. 1 is a block diagram showing a liquid crystal display device according to the related art. In FIG. 1, a liquid crystal display (“LCD”) device includes a liquid crystal panel 1, a gate driver 4, a data driver 6, a timing controller 7, a backlight unit 8 and a source voltage generator 9. The liquid crystal panel 1 includes a plurality of thin film transistors (not shown) disposed in a matrix thereon. The gate driver 4 controls input of a data signal into the liquid crystal panel 1, and the data driver 6 inputs the data signal to the liquid crystal panel 1. The timing controller 7 controls a timing of the gate driver 4 and the data driver 6.
The backlight unit 8 is disposed under and supplies light to the liquid crystal panel 1. Further, the backlight unit 8 includes a backlight lamp 8a emitting light and a backlight driver 8b controlling the backlight lamp 8a. The source voltage generator 9 supplies source voltages to the gate driver 4, the data driver 6, the timing controller 7 and the backlight unit 8. The source voltage generator 9 is formed on a printed circuit board (“PCB”). Although not shown, the backlight lamp 8a includes one of at least one fluorescent lamp and a plurality of light emitting diodes (“LEDs”).
Each of the thin film transistors (“TFTs”) uses hydrogenated amorphous silicon (“a-Si:H”) in a semiconductor layer. Hydrogenated amorphous silicon yields higher productivity while easily fabricated on a large sized substrate. In addition, since hydrogenated amorphous silicon is deposited at a temperature less than about 350° C., a glass substrate of low cost can be used. Accordingly, hydrogenated amorphous silicon is used mainly in a TFT, which is referred to as an amorphous silicon thin film transistor (“a-Si TFT”).
However, since hydrogenated amorphous silicon has a disordered atomic arrangement, weak silicon-silicon (“Si—Si”) bonds and dangling bonds exist in hydrogenated amorphous silicon. These types of bonds become metastable when light or an electric field is applied to hydrogenated amorphous silicon. As a result, such metastability makes the TFT unstable. Specifically, electric characteristics of hydrogenated amorphous silicon are degraded due to light irradiation. Furthermore, a TFT using hydrogenated amorphous silicon is difficult to be implemented in a driving circuit due to degraded electric characteristics such as a low field effect mobility between about 0.1 cm2/Vsec to about 1.0 cm2/Vsec, and poor reliability.
Accordingly, the substrate including a-Si TFTs is connected to a printed circuit board (“PCB”) using a tape carrier package (“TCP”) that has a driving integrated circuit (“IC”). The driving IC and its packaging increase production cost of the LCD device. Additionally, as the resolution of a liquid crystal panel for an LCD device increases, a pad pitch between gate pads or between data pads of the substrate including the a-Si TFT becomes smaller. Thus, bonding between the TCP and the substrate including the a-Si TFT becomes harder.
To solve these problems, a polycrystalline silicon thin film transistor (“p-Si TFT”) is suggested. Due to a higher field effect mobility of a p-Si TFT as compared to an a-Si TFT, a driving circuit can be integrated on a substrate including the p-Si TFT, such that a driving element and a switching element are simultaneously formed. Accordingly, the TCP is not needed and the production cost is reduced. Moreover, a driving system may be integrated in the liquid crystal panel, and an LCD device where a driving system is integrated in a liquid crystal panel may be referred to a system on panel (“SOP”) type LCD device.
FIG. 2 is a block diagram showing a liquid crystal display device using a polycrystalline silicon thin film transistor according to the related art. In FIG. 2, a liquid crystal display (“LCD”) device includes a liquid crystal panel 10 having a liquid crystal layer interposed between two substrates. The liquid crystal panel 10 includes a display area 12 for displaying images and a non-display area 13 defined therein. A gate line “GL” and a data line “DL” crossing each other are formed in the display area 12. In addition, a thin film transistor (“TFT”) “T” is connected to the gate line “GL” and the data line “DL.”
A gate driver 14 and a data driver 16 are formed in the non-display area 13. The gate driver 14 and the data driver 16 respectively receive a gate signal and a data signal from an exterior system (not shown) and control the TFT “T” in the display area 12 through the gate line “GL” and the data line “DL,” thereby changing light transmittance of the liquid crystal layer. Even though not shown in FIG. 2, a timing controller and a source voltage generator are formed on a printed circuit board (“PCB) and connected to the liquid crystal panel 10. Moreover, a backlight unit is disposed under the liquid crystal panel 10.
Since a backlight unit of an LCD device emits light of constant intensity, a display quality of the LCD device is deteriorated according to ambient brightness. When the backlight unit emits light of relatively low intensity, images displayed in the LCD device are rarely recognized under a circumstance of high ambient brightness. In addition, when the backlight unit emits light of relatively high intensity, power is wasted under a circumstance of low ambient brightness because light of relatively low intensity is enough to display recognizable images.